This invention relates to an accelerometer structure and force-balance feedback system for measuring strong-motion seismic events, and is particularly useful for measuring earthquake vibrations in buildings, bridges and other structures, such vibrations being substantially within a frequency band of 0.1 Hz to 50 Hz, and having amplitude levels up to about 5G peak.
Force-balance accelerometers and seismometers are well known in the art, and generally are of the type which produces a signal proportional to displacement of a proof mass and then feeds back components of the signal to a winding to at least partially oppose such displacement. A few of them have used a combination of a piezoelectric or ferroelectric transducer to generate the signal and a winding to which components the signal are fed back.
U.S. Pat. No. 4,186,324 to Hartzell suggests this general concept by showing a piezoelectric transducer means 10 and a torque coil 14, both carried by a bendable arm which cooperates with a position detector. It is the function of the position detector plus the piezoelectric transducer to deliver error signals to an amplifier system that generates a restorative current and delivers it to the torque coil to return the mass to an initial null position. The structure of the accelerometer and the electrical circuitry however are different from the present invention and lack certain advantages that will be set forth below.
U.S. Pat. No. 3,229,531 to Stiles employs piezoresistive elements connected in series with a battery to modulate a current through a coil in a force-balance accelerometer to provide its restoring force. However, this arrangement draws current continuously from the battery and therefore would not be acceptable for use in an accelerometer that must wait for seismic events over long periods of time in an isolated location where low power drain is vitally important.